1. Field of Invention
The present invention provides a novel class of borane adducts with selected tertiary diisopropylalkylamines, specifically diisopropylalkylamines wherein the alkyl group is a branched-chain C.sub.3 -C.sub.7 alkyl, and their use in the hydroboration and reduction of organic compounds. The novel borane-amine adducts of this invention have a number of advantages over the presently available agents.
2. Prior Art
Borane adducts with amines are versatile reagents exhibiting many different properties as compared to the metal borohydrides. For example, they are soluble in a variety of solvents, including hydrocarbons or even water, and in some cases can be used in an acidic medium. Many adducts have been synthesized. See for example, Long, L.H. in W.J. Mellor A Comprehensive Treatise on Inorganic and Theoretical Chemistry; Longman: London, 1981, Supplement Vol. 5, Part B1, p 1.; and Meller, A. In Gmelin Handbook of Inorganic and Organometallic Chemistry; Springer: Berlin, 1992, 4th Supplement, Vol. 3, p 1. Several are commercially available. They find various uses, e.g., as fuel additives, polymerization catalysts, polymer stabilizers and stain removers, in metal plating and in the dye and pharmaceutical industries. See Lane, C.F. Aldrichimica Acta 1973, 6, 51. Most of these applications are based on their reducing properties.
In contrast, the use of borane-amine adducts for hydroboration is rather limited due to strong complexation, which renders their reactivity low as compared to the weak borane adducts with ethers and Gulfides. For example, boranetriethylamine does not hydroborate 1-octene at room temperature and only very slowly in refluxing tetrahydrofuran (THF). See Brown, H.C. et al. Inorganic Chem. 1984, 23, 2746.
Amines as borane carriers offer significant advantages often giving adducts of low sensitivity to moisture and air and readily soluble in representative solvents. Environmentally important is an easy recovery of the amine from the hydroboration products, making possible its ready recycling. The significance of these factors becomes apparent with the growing importance of diborane for the synthesis of pharmaceuticals and other valuable compounds. However, the well established reagents, boranetetrahydrofuran and borane-dimethylsulfide (BMS) suffer a number of disadvantages for large-scale commercial applications as discussed below.
Borane-tetrahydrofuran is a valuable reagent for the hydroboration of olefins and for the reduction of organic compounds. It suffers from the disadvantage in that the solutions are unstable over a period of time. U.S. Pat. No. 3,882,037 discloses stabilized borane-tetrahydrofuran solutions which permit storage of such solutions for relatively longer periods of time. However, the inherent availability only as a relatively dilute solution in tetrahydrofuran largely restricts its use to tetrahydrofuan solutions and poses a drawback to commercial use of this reagent.
Borane-methyl sulfide (BMS) is much more stable than borane-tetrahydrofuran and is widely used for both hydroboration and reduction [See Burg et al., J. Am. Chem. Soc. 76, 3307 (1954) and Coyle et al., J. Am. Chem. Soc. 81, 2989 (1959)]. However, it suffers from the serious disadvantage in that it yields a product which contains free dimethyl sulfide. The free dimethyl sulfide is highly volatile, b.p. 38.degree. C., flammable and has a very noxious odor. Moreover, it is not soluble in water, so it cannot be disposed of by washing it away with water which poses a serious environmental problem.
Borane-1,4-thioxane (U.S. Pat. No. 4,298,750) is another valuable hydroboration agent. It has both lower volatility and milder odor than dimethyl sulfide. It has a limited solubility in water and can be easily oxidized to the corresponding sulfoxide, which is miscible in water. This agent is a liquid, 8M in BH.sub.3, stable over prolonged periods. Unfortunately, this commercially available reagent is relatively costly compared to borane-tetrahydrofuran and borane-dimethyl sulfide.
The growing importance of borane reagents for the synthesis of pharmaceuticals and other compounds and the problems associated with other well established borane adduct hydroboration agents, e.g., low concentration and stability, high volatility, flammability, unpleasant odor, as discussed above, create a need for easy to handle, stable and environmentally benign hydroborating agents as discussed specifically below.
Thus, the search continues for effective, versatile borane derivatives which are as effective as the commercially available reagents but which overcome the disadvantages of noxious odor, expense, volatility, and lack of water solubility. The amines of the present invention are well suited for that purpose. They have an agreeable odor, form neat adducts, are highly concentrated in borane, are soluble in various solvents, and the amine can be readily removed and recovered from hydroboration products.
Although many borane-amine adducts are known, almost all amines used for their preparation are relatively unhindered and hence strongly complexing, producing borane adducts which exhibit a low reactivity for hydroboration at room temperature. See C.F. Lane, Aldrichimica Acta, 6, 51 (1973). On the other hand, it has been reported that highly hindered 1,2,2,6,6-pentamethylpiperidine does not form a borane adduct. See Flores-Parra, N. et al., Tetrahedron Lett., 47, 6903 (1991).
It is wholly surprising that the borane- N,N-diisopropylalkylamines of the present invention are valuable hydroborating agents since the ethyl derivative is a hindered amine widely used as a proton scavenger [Raber, D.J. et al., Tetrahedron Lett., 4741 (1971)] but which forms a borane adduct that only slowly hydroborates 1-octene in tetrahydrofuran at room temperature. The compounds of this invention, on the other hand, hydroborate 1-octene in tetrahydrofuran at room temperature in less than anhour, and in most cases in 15-30 minutes.